Saturday, June 20, 2009

A Transformer-Coupled Booster Amplifier for the AN214

Due to it’s inherently low power output design, does addition of a transformer-coupled booster amplifier to an AN214-based amplifier aid or hinder it’s sound quality?

By: Vanessa Uy

Given it’s inherently low power output – between 4 to 5 watts into an 8-ohm load. It was therefore inevitable to find a way of increasing the “meager” output of the AN214 IC-based audio amplifier without sacrificing it’s inherently good sound quality (on the cheap?). I wonder how many ways were “engineered” during the past 30 + years or so history of this device. Surprisingly, a consensus was reached of choosing a “primitive” transformer-coupled PNP transistor-based design that probably dates from the 1950s.

The design configuration of the transformer-coupled booster amplifier used to increase the 5-watt power output of the AN214 IC-based power amplifier for all intents and purposes resembles that of a transformer-coupled 300B-based push-pull tube power amplifier circuit. Except that the 300B tubes (or valves as they say in Merry Old England) were replaced by MJ2955 PNP power transistors in a TO-3 package. From my point of view, this transistor-based transformer-coupled booster amplifier circuit was probably based on the first transistor-based audio power amplifier designs first published in the US-based audio electronics magazine called AUDIO. Probably those issues dated between January 1960 to December 1961 - i.e. the Golden Age of Stereo.

During my experimentation of the MJ2955-based transformer-coupled booster amplifier – which to all intents and purposes is a continuation of the experiment done by my older audio-buddies during the last 25 years or so – I did optimize the input transformer winding design. By eliminating stray capacitance of the input transformer / driver transformer winding and optimizing it for a near-perfect square wave performance and transfer function characteristics. I did manage to reduce the booster amplifier’s recommended negative feedback level, which only improved the sound quality to no end.

Compared to its 300B tube-based sibling sound quality wise, it was a close match. Although the 300B-based push-pull amplifier did manage to display irreproachable beauty where it excels. Like in the Middle-C region of the musical performance – i.e. upper bass and lower midrange region of the audio spectrum. And even though it was criticized during the 1980s that transistor-based transformer-coupled booster amplifiers are notorious for having high total harmonic distortion (THD) in comparison to a full complementary direct-coupled solid-state power amplifier. But if you wind the transformers properly by eliminating stray capacitance – especially the input / driver transformers, the inherently high THD (total harmonic distortion) of transformer-coupled solid-sate booster amplifiers can be dramatically reduced. Although high-quality audio frequency transformers – driver transformers and output transformers - that are wound at artisan levels to optimize their square-wave performance and optimize their transfer-function characteristics across the audio spectrum are magnitudes more expensive in comparison to matched pairs of high-power output transistors.

The bad news is – from a perfectionist audiophile’s perspective – a lot can go wrong, sound quality wise, in the actual construction of full complementary direct-coupled solid-state power amplifiers. Especially if the “engineer” doesn’t trust their own ears and chose to trust on measuring instruments instead could result in a sluggish sounding power amplifier with unlimited reserves of power - but one that is sorely lacking in the pace, rhythm, and timing department. That's why I really hate musically illiterate audio engineers. Looks like the old-school wisdom gained by audio engineers during the Golden Age of Stereo still applies well into the 21st Century. And it also provides a low-cost route to audio nirvana via the AN214 IC-based audio power amplifier route. Ain’t it fun to be an audio cheapskate?

Wednesday, June 17, 2009

What in the World is an AN214 IC?

In spite of it’s age – and apparent manufacturing extinction – are AN214 IC-based audio power amplifiers still “noteworthy” in our 21st Century hi-fi world?

By: Vanessa Uy

If you asked me – or any of my older audio-buddies – about AN214 IC-based audio amplifiers, they do admit – albeit reluctantly – to have “once upon a time” taken this integrated circuit route to audio nirvana. Given that this low powered audio-amplifier-on-a-chip is about as technologically advanced as a cassette tape when viewed from our 21st Century perspective, does it still deserve serious discussion? Though the short answer by audiophile die-hards lucky enough to stock significant quantities of the AN214 IC is: “you betcha!”

Part of the allure and mystique of the AN214 integrated circuit-based audio power amplifier is that manufacturer authorized and approved – i.e. reliable – spec sheets and application notes are few and far between. In my opinion, it is safe to say that anyone who has seen first hand “proper” AN214 IC spec sheets – especially the device’s slew rate and power output rating – are either: a) had forgotten about it due to illicit drug use during the past 25 years. b) Have gallantly gave their lives during Operation Desert Storm, or more likely c) had joined a technology-rejecting / technology-abhorring religious cult.

But what everyone lucky enough to remember the virtues of the AN214 power amplifier during it’s heyday is that it has always been inexplicably linked with the ubiquitous back in the 1970s and 1980s Pioneer in-car stereo cassette tape deck and FM tuner combo. Which from an electronic engineer’s standpoint is inevitable given that both live in the 12-volt world of car audio. Although I’m lucky enough to have heard first hand Pioneer’s 8-track version of this in-car tape and FM tuner combo that sounds as if it was designed by famed recording engineer Bill Laswell due to it’s legendary ability to reproduce recorded music with tons of electric bass. I f you’re an unabashed reggae music and dub fan lucky enough to demo this legendary Pioneer in-car 8-track tape and FM stereo combo. Buying it would be the next best thing to hanging out with Bill Laswell in the recording studio capturing your favorite bass-based musician on analogue tape.

Although the AN214 IC audio amplifier was famed in its heyday for “heroically” bringing out the musicality of the cassette tape medium despite of it’s “engineering” limitations. This 5 watt (according to bench test measurements, although it sounds apparently more powerful in real life use) integrated circuit-based power amplifier has still so much to give in our wide bandwidth wide dynamic ranged hi-fi world. Even in the era of DVD Audio and SACD whose bandwidth could easily stretch to 100,000 Hz. Compared to CD’s 22,050 Hz Nyquist Frequency criterion limitation – never mind cassette tapes, which have a hard time reaching past 15,000 Hz.

Given that the first decade of the 21st Century is almost over, I find it real surprising than an integrated circuit-based low-power amplifier whose heyday pre-dates the discovery of the AIDS virus can still hold its own in 2009. If any of you out there has something extra to add to my woefully inadequate knowledge to the electronic engineering aspects of the AN214 IC-based power amplifier, please drop me a line. Your help is greatly appreciated. Especially the slew rate part of this venerable device.